Social Question

Is there a gap in the sizes of animals?

Most animals are, from our perspective, fairly small – rodents, weasels, most birds, most lizards and most fish. Then there are animals that are large – humans, big cats, bears, deer, alligators, elephants, rhinos, ostriches and white sharks. There does not seem to be much in the middle, from about 40 pounds to 100 pounds. You can see this really clearly if you look at a single type of animal. In the cat family, there is a pretty big jump in size from bobcat to cougar.

Is this observation correct and, if so, why should it be? I am thinking that there is something similar among plants. There is a pretty big gap between the sizes of bushes and of trees.

19 Answers

A few animals spring to mind- the Tapir, some antelopes are small-ish, and there is a breed of wild cat that is about fifty pounds or so. Perhaps it’s because small animals can hide, big ones intimidate but those in the middle need more specialized evolutionary traits to help them survive and thus, it knocked out a lot of the ‘misfires’ over time. Like an octopus- how many failed variations before we got that miraculous creature?

The ones that fall in the range tend to be in the upper part, from about 80 to 100 pounds. Wild goats and sheep I still think that the number of animals in the 40 to 100 pound range is fairly small. It would be interesting to see a curve of number of animals (in terms of number of species and number of individuals) versus weight. Insects would have to be left out, but there could be a separate curve for them, which might tell a lot, since proportionately speaking, insect sizes cover a far greater range than larger animals.

There’s also a large gap like this when it comes to marine creatures. There are huge marine mammals like whales, and the next smallest thing would be around the size of a dolphin or manatee. Not a whole lot in between. He’s a size chart for marine mammals as an example.

Well, the Insect kingdom is completely separate from the Animal kingdom, so they wouldn’t even be considered.

Still, I think your premise is flawed. There are many, many animals that range 40 to 100 lbs, even if you didn’t “count” domestic dogs (though I don’t know why you wouldn’t. Sure, they were bred that way, but everything alters through breeding.)

There is no gap in the fossil record of increasing complexity and size within the animal kingdom. The apparent gap is in what remains surviving today. That gap probably relates to predation by large carnivores including, of course, humans.

@marinelife , @Randy , Thanks for your examples. I was surprised by some of them. I did not realize African wild dogs and sea otters were that large or that cheetahs and snapping turtles were that small.

Note though that there is an error by an order of magnitude in the weight of a hooded vulture. Flying birds should be considered separately. There is a wide range of weight ratios but not much range in actual weight. An eagle only weighs about 10 to 14 pounds.

Okay, the gap is not as big as I thought and for mammals it may not even exist, but consider some other animals. Komodo dragons are the largest lizards, getting up to 200 pounds. I don’t think there are any iguanas that get anywhere close to that. Similarly, I can’t think of any snakes that get anywhere near the size of anacondas or pythons. Newts are generally very small, but there is one in China that gets to be six feet long. Most octopus species are very small. I believe that there is just the one giant octopus that gets that big. I think the same thing holds for squids.

Well maybe, but then there is still the question as to why certain families can compete at the large and small sizes but not at the intermediate ones. I think that in the case of the Komodo dragon, geographic isolation is a factor and the same may hold for the large Australian snakes. Somebody really should do a study of animal size distribution.